Circuit for sinking current with near-ground voltage compliance

ABSTRACT

The present invention is a closed-loop peak detection circuit comprising switching means, comparing means, control means, two current sources, and a holding capacitor. The switching means selectively provides one of a plurality of input signals to the comparing means. The control means is coupled to the comparing means. The control means receives first and second control signals for selecting one of three modes: reset, peak detect, and hold. First and second current sources are coupled to the control means. A capacitor is coupled to the first and second current sources for generating an output signal. The output signal is feedback coupled to the comparing means. The comparing means determines when one of the plurality of input signals exceeds the output signal. The control means enables and disables the current sources in response to the comparing means and to the first and second control signals. In reset mode, the second current source discharges the capacitor until the output voltage is equal to a baseline voltage V REF . In peak detect mode, the first current source charges the capacitor when the instantaneous value of the output signal is less than the input signal. The peak detector uses the input signal to pump up the output voltage to the most positive value of input signal. In hold mode, the current sources are disabled. The capacitor maintains a nearly constant amount of charge. Thus, the output voltage is equal to the peak voltage of the input signal stored on the capacitor.

This is a divisional of application Ser. No. 08/140,757, filed Oct. 20,1993, now U.S. Pat. No. 5,428,307.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to the field of peak detection.

2. Background Art

Peak detectors are devices for determining the maximum (peak) amplitudeof an input signal. Such circuits typically comprise a holding capacitorfor storing charge from the input signal and control circuitry to governapplication of the input signal to the holding capacitor. Thus, thecontrol circuit controls charging and discharging of the holdingcapacitor. In conjunction with the control circuit, the holdingcapacitor is charged up to store a voltage that increases toward thepeak value of the input signal.

FIG. 1 is a diagram illustrating a prior art peak detector comprisingNPN transistor Q102 and holding capacitor C102. Supply voltage V_(CC) isprovided to the collector of transistor Q102. Input signal 100 iscoupled to the base of transistor Q102. The emitter of transistor Q102is coupled to an external holding (memory) capacitor C102 through inputpad IP102. The second terminal of capacitor C102 is coupled to ground.The output voltage V_(OUT) is taken across capacitor C102 with respectto ground. The circuit illustrated in FIG. 1 is an open-loop peakdetector.

In FIG. 1, when input signal 100 becomes more positive than the sum ofoutput voltage V_(OUT) across capacitor C102 and the base-emittervoltage V_(BE) of transistor Q102, the emitter of transistor Q102 ispulled up. Transistor Q102 sources current to holding capacitor C102.This current charges up capacitor C102 until the input signal 100decreases below the instantaneous value of output voltage V_(OUT) plusthe base-emitter voltage V_(BE) of transistor Q102. When the inputsignal 100 provided to the base of transistor Q102 drops below thislevel, the emitter-base junction of transistor Q102 is reverse biased.In this state, current is not conducted through transistor Q102 tocapacitance C102. Thus, capacitor C102 holds the most positive value ofthe input signal 100 less the V_(BE) drop of transistor Q102 thatcapacitor C102 charged to.

A disadvantage of this prior art circuit is that the output voltagecannot reach the peak amplitude of the input signal. The output voltageV_(OUT) stored on capacitor C102 has a maximum voltage level of onebase-emitter voltage drop less than the peak input signal 100. Also, thebase-emitter voltage drop of transistor Q102 makes the prior art peakdetector insensitive to peaks of the input signal 100 less thanapproximately 0.6 volts above the instantaneous voltage level of theoutput signal V_(OUT).

Another disadvantage of this prior art circuit is inaccuracy in theoutput voltage due to variations in base-emitter voltage of thetransistor. The variations in base-emitter voltage of transistor Q102are caused by temperature and current variations. Thus, the offset ofthe peak voltage across capacitor C102, due to the base-emitter voltageof transistor Q102, varies over temperature and device beta.

FIG. 2 is a diagram illustrating another prior art peak detector. Thisclosed-loop peak detector comprises an operational amplifier OP202,diode D202 and holding capacitor C102. Input signal 100 is coupled tothe non-inverting input of operational amplifier OP202. The output ofoperational amplifier OP202 is coupled to a first terminal of diodeD202. The second terminal of diode D202 is coupled to a first terminalof capacitor C102 and to the inverting input of operational amplifierOP202. The second terminal of capacitor C102 is coupled to ground. Theoutput voltage V_(OUT) is provided across capacitor C102.

Input signal 100 drives operational amplifier OP202. In turn,operational amplifier OP202 controls holding (memory) capacitor C102through diode D202. The closed-loop feedback of output voltage V_(OUT)to operational amplifier OP202 causes operational amplifier OP202 toprovide a voltage at its output that is one diode drop above outputvoltage V_(OUT). The operational amplifier OP202 acts as a unity gainbuffer that can only source current due to diode D202. When input signal100 drops below output voltage V_(OUT), diode D202 is reverse-biased sothat no charging current is provided to capacitor C102.

When input signal 100 exceeds the output voltage V_(OUT), operationalamplifier OP202 generates a voltage to forward bias diode D202. Withdiode D202 forward biased, operational amplifier OP202 sources a currentto charge capacitor C102. The charging current increases the outputvoltage V_(OUT) across capacitor C102 until it matches the input signal100. The closed-loop of this prior art circuit interrupts the chargingcurrent through diode D202 when the instantaneous output voltage V_(OUT)equals the input signal 100. In this prior art circuit, the effect ofthe offset voltage of diode D202 is reduced. However, this prior artpeak detector has several disadvantages.

A disadvantage of the prior art is due to the finite slew-rate of theoperational amplifier in the peak detector. The voltage output ofoperational amplifier OP202 goes into negative saturation when the inputsignal is lower than the output voltage V_(OUT). The finite slew-rateprevents the voltage generated by operational amplifier OP202 fromaccurately following higher frequency input signals 100 when inputsignal 100 exceeds the output voltage V_(OUT).

Another disadvantage of the prior art is "droop" of the output voltagedue to bias and leakage currents. The input bias current of operationalamplifier OP202 discharges capacitor C102 producing a decrease in outputvoltage V_(OUT). Similarly, the leakage current of diode D202 alsoproduces droop in the output voltage V_(OUT).

Yet another disadvantage of the prior art is poor noise rejection due tothe diode. Transients in input signal 100 that exceed the instantaneousvalue of output voltage V_(OUT) force diode D202 to conduct. Thisproduces an error in the output voltage because the charging currentsourced by operational amplifier OP202 increases the output voltageV_(OUT). Thus, the prior art provides poor rejection of noise.

SUMMARY OF THE PRESENT INVENTION

The present invention is related to the field of peak detection. Theclosed-loop circuit comprises: switching means, comparing means, controlmeans, two current sources, and a holding capacitor. The switching meansreceives a plurality of input signals. The switching means selectivelyprovides one of the plurality of input signals to the comparing means.Control means is coupled to the comparing means. The control meansreceives first and second control signals for selecting one of threemodes: reset, peak detect, and hold. First and second current sourcesare coupled to the control means. A capacitor is coupled to the firstand second current sources for generating an output signal. The outputsignal is feedback coupled to the comparing means. The comparing meansdetermines when one of the plurality of input signals exceeds the outputsignal. The control means enables and disables the first and secondcurrent sources in response to the comparing means and to the first andsecond control signals.

In the reset mode, the second current source draws current from thecapacitor until the output voltage of the peak detector is discharged toa baseline voltage V_(REF). The present invention maintains the outputvoltage at the constant level of V_(REF). In the peak detect mode, thefirst current source provides current to charge the capacitor until theoutput voltage is equal to the instantaneous value of the input signal.The present invention uses the input signal to pump up the outputvoltage to the most positive value of input signal, In the hold mode ofthe present invention, the first and second current sources aredisabled. Current is not sourced to or drained from the capacitor. Thecapacitor maintains a nearly constant amount of charge. Thus, the outputvoltage is equal to the peak voltage of the input signal stored on thecapacitor.

The comparing means of the present invention comprises a current sinkhaving near-ground voltage compliance. The circuit for sinking a currentwith near-ground voltage compliance comprises a diode, two transistors,and a Norton amplifier. A diode is coupled to a supply voltage. A firsttransistor is coupled to ground. A differential pair is coupled betweenthe diode and the first transistor. A second transistor is coupled tothe differential pair and the diode for detecting a first currentprovided by the first transistor. The second transistor provides asecond current in response to the first current. The Nortonamplification means is coupled to the first transistor for receiving thesecond current and a reference current. The Norton amplification meanscontrols the first current through the first transistor in response tothe second current and the reference current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a prior art (open-loop) peak detector;

FIG. 2 is a diagram illustrating another prior art (closed-loop) peakdetector;

FIGS. 3A-3C are diagrams illustrating the present invention in threemodes of operation;

FIG. 4 is a timing diagram illustrating the three modes of the presentinvention;

FIG. 5 is a timing diagram illustrating peak detection according to thepresent invention;

FIG. 6 is a differential amplifier with an idealized current sink(open-loop);

FIG. 7 is a practical prior art realization of the current sink(open-loop);

FIG. 8 is a diagram illustrating a current sink with near-groundcompliance range according to the present invention; and,

FIGS. 9(a) and 9(b) are alternative embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

A closed-loop peak detector is described. In the following description,numerous specific details, such as number and nature of externalsignals, transistors, etc., are described in detail in order to providea more thorough description of the present invention. It will beapparent, however, to one skilled in the art, that the present inventionmay be practiced without these specific details. In other instances,well-known features have not been described in detail so as not tounnecessarily obscure the present invention.

CLOSED-LOOP PEAK DETECTOR

FIG. 3A is a diagram illustrating the preferred embodiment of thepresent invention comprising switch 302, comparator 304, AND gates306-308, current sources I310-I312 and holding capacitor C102. Inputsignal 100 is provided to a first terminal of switch 302. Constantreference voltage V_(REF) is provided to a second input terminal ofswitch 302. The output of switch 302 is coupled to the non-invertinginput of comparator 304. The output of comparator 304 is coupled to theinput of AND gate 306 and the inverting input of AND gate 308. A peakdetection control signal PKDET is provided to a second input of AND gate306. A reset control signal RESET is provided to a second input of ANDgate 308. The output of AND gate 306 is provided to current source I310.The output of AND gate 308 is provided to current source I312. A firstterminal of current source I310 is coupled to voltage supply V_(CC). Thesecond terminal of current source I310 is coupled to a first terminal ofcurrent source I312, the inverting input of comparator 304 and a firstterminal of capacitor C102. The second terminals of current source I312and capacitor C102 are coupled to ground. The output voltage V_(OUT) isprovided from the node between current sources I310 and I312.

As illustrated in FIG. 3, the PKDET and RESET signals are provided tothe peak detector circuit of the present invention. The circuit operatesin three modes: reset, peak detect and hold. The peak detector comprisesa comparator 304 having a non-inverting input coupled through switch 302to input signal 100 (which can be a full wave rectified signal) and areference voltage V_(REF). The output of comparator 304 is provided toAND gates 306 and 308. The upper AND gate has the PKDET signal as aninput, and its output controls current source I310 (IPKDET). Similarly,the RESET signal is provided to the AND gate 308 that controls currentsource I312 (I_(RESET)). The output voltage V_(OUT) is taken from thenode between the two current sources I310 and I312 and is coupled in afeedback loop to the inverting input of comparator 304. Control block320 comprises AND gates 306 and 308.

FIG. 3A illustrates the present invention in the reset mode. When in thereset mode, the PKDET signal is logic low (0) and the RESET signal islogic high (1). In this mode, switch 302 couples the non-inverting inputof comparator 304 to reference voltage V_(REF). Because the peakdetection signal PKDET is low, AND gate 306 is disabled. Thus, currentsource I310 (I_(PKDET)) is disabled.

Initially, when capacitor C102 is completely discharged, the output ofcomparator 304 is high. The high output of comparator 304 is provided toAND gate 308. Because the input of AND gate 308 is negated, its outputis low, thereby turning off current source I312. This keeps capacitorC102 in a discharged state, and output voltage V_(OUT) is nearly ground.

When output voltage V_(OUT) is greater than voltage reference V_(REF),comparator 304 outputs a logic low. In response to the low output ofcomparator 304, AND gate 308 produces a high output. The high output ofAND gate 308 enables current source 1312 (I_(RESET)). Current sourceI312 draws current from capacitor C102 until the output voltage V_(OUT)of the peak detector is discharged to the baseline voltage V_(REF)applied to switch 302. When this occurs, the output of comparator 304goes high and current source I312 is disabled. This maintains the outputvoltage V_(OUT) at the constant baseline level of V_(REF).

FIG. 3B is a diagram illustrating the peak detection mode of the presentinvention. When in this mode, the PKDET signal is high and the RESETsignal is low. Also, input signal 100 is provided to comparator 304 byswitch 302. Thus, AND gate 308 is disabled, thereby disabling currentsource I312. When input signal 100 is lower than the voltage V_(OUT)stored across capacitor C102, the output of comparator 304 is low. Theoutput of AND gate 306 disables current source I310 that chargescapacitor C102.

When input signal 100 exceeds output voltage V_(OUT), the output ofcomparator 304 goes high. This enables current source I310. Currentsource I310 charges capacitor C102 until the input signal 100 decreasesbelow the instantaneous level of output voltage V_(OUT). Comparator 304is a MOSFET input comparator which has no bias current. It compares theinput signal 100 with the voltage V_(OUT) using closed loop feedback.

When the output of comparator 304 is high, current source I310(I_(PKDET)) provides current to charge capacitor C102 until the outputvoltage V_(OUT) is equal to the instantaneous value of the input signal100. The output voltage V_(OUT) across capacitor C102 is slew-ratelimited. Therefore, the present invention requires a series of positivelobes of input signal 100 to pump up voltage V_(OUT) to the mostpositive value of input signal 100. Thus, in peak detect mode, currentsource I310 successively charges capacitor C102 when input signal 100exceeds voltage V_(OUT), thereby increasing voltage V_(OUT).

FIG. 3C illustrates the hold mode of the present invention. The PKDETand RESET signals are both low. In this mode, switch 302 couplesreference voltage V_(REF) to the non-inverting input of comparator 304.With the PKDET and RESET signals low, current sources I310 and I312 aredisabled. Thus, current is not sourced to or drained from capacitorC102, and capacitor C102 maintains a nearly constant amount of charge.Thus, the output voltage V_(OUT) is equal to the peak voltage of theinput signal 100 stored in capacitor C102.

It should be apparent to one skilled in the art that a fourth possiblemode exists for the circuit described: PKDET=1 and RESET=1. In thismode, both current sources I310 and I312 are enabled. When switch 302provides voltage V_(REF) to the non-inverting input of comparator 304, asecond reset mode with bi-directional current drive to capacitor C102 isactivated. This mode provides a capacitor reset level closer to V_(REF),which is more desirable in some applications.

TIMING DIAGRAM ILLUSTRATING THREE MODES

FIG. 4 is a timing diagram illustrating the input voltage applied to thenon-inverting input of comparator 304 and the corresponding outputvoltage signal V_(OUT). The input voltage at the non-inverting input ofcomparator 304 is indicated by a dotted line. During times 0 to T₁, thePKDET signal is low and the RESET signal is high. Initially, capacitorC102 is discharged and, thus, the output signal V_(OUT) has a level of 0volts (indicated by a solid line). Because switch 302 provides voltageV_(REF) to the non-inverting input of comparator 304, the input voltageis equal to V_(REF). As indicated in FIG. 4, current sources I310 andI312 are disabled. The output voltage V_(OUT) is maintained near ground.

Between times T₁ and T₃, the present invention operates in the peakdetect mode. As illustrated in FIG. 3B, the PKDET signal is high and theRESET signal is low. In this mode, the non-inverting input of comparator304 is coupled to input signal 100. In FIG. 4, the input signal 100(indicated by a dashed line) is a full-wave-rectified voltage signal.The diagram illustrates six lobes of input signal 100 during thisinterval.

At time T₁, the output voltage V_(OUT) is equal to 0 volts. Betweentimes T₁ and T₂, the input signal 100 provided to the non-invertinginput of comparator 304 is greater than the output signal V_(OUT). Thus,the high output of comparator 304 enables current source I310. Currentsource I310 charges capacitor C102. In FIG. 4, the output voltageV_(OUT) increases linearly during times T₁ and T₂ in response to thethree lobes of input signal 100.

At time T₂, the instantaneous voltage V_(OUT) rises to the lowest levelof input signal 100. The output of comparator 304 goes low in response.The low output of comparator 304 disables current source I310. Becausecurrent sources I310 and I312 are disabled, the output voltage V_(OUT)of the peak detector is held at a constant level. The output voltageV_(OUT) remains constant until the voltage applied to comparator 304exceeds the output voltage. Between times T₂ and T₃, the output voltageV_(OUT) alternately increases by charging capacitor C102 using currentsource I310.

At time T₃, the peak detector switches to the hold mode by setting lowthe PKDET and RESET signals. Both current sources I310 and I312(I_(PKDET) and I_(RESET)) are disabled. During the hold mode, switch 302is coupled to constant voltage reference V_(REF) as indicated by theconstant level (indicated by a dotted line). In FIG. 4, between times T₃and T₄, the output voltage V_(OUT) is held at a constant level V₁ (wherevoltage 0<V₁ <V_(MAX)).

At time T₄, the peak detector of the present invention operates in thereset mode. The PKDET signal is low and the RESET signal is high.Setting the PKDET signal to zero disables current source I310. Switch302 couples the non-inverting input of comparator 304 to the constantreference voltage V_(REF). As indicated in FIG. 4, the output voltageV_(OUT) (solid line) has a value of V₁. It is greater than the inputvoltage of comparator 304 (indicated by a dotted line) equal to V_(REF).Comparator 304 outputs a logic low, thereby. enabling current sourceI312 (I_(RESET)) which discharges capacitor C102. The output voltageV_(OUT) decreases linearly from a level of V₁ to V_(REF).

At time T₅, V_(OUT) reaches the reference voltage V_(REF) coupled tocomparator 304. In turn, the output of comparator 304 goes high, therebydisabling current source I312. Between times T₅ and T₆, the outputvoltage V_(OUT) has a constant level of V_(REF).

At time T₆, the present invention operates in the peak detect mode asillustrated in FIG. 3B. Between times T₆ and T₇, input signal 100 iscoupled to the non-inverting input of comparator 304. In FIG. 4, thereare six lobes of the input signal 100 during this interval. When inputsignal 100 exceeds the output signal V_(OUT), the output of comparator304 goes high. This enables peak detect current source I310. Currentsource I310 charges capacitor C102 until the input signal 100 decreasesbelow the instantaneous value of the output voltage V_(OUT). Prior totime T₇, current source 310 charges output voltage V_(OUT) to the peakvalue of the input signal 100 (as illustrated in FIG. 4).

At time T₇, the present invention is operated in the hold mode where thePKDET and RESET signals are low. This disables current sources I310 andI312 which prevents the sourcing and sinking of current to holdingcapacitor C102. Thus, in FIG. 4, the output voltage V_(OUT) remains atits peak value of V_(MAX) after time T₇.

The present invention provides closed loop peak detection with lowoffset voltage, no amplifier slew-rate limitations, and with enhancednoise rejection capability. The peak voltage stored on holding capacitorC102 is not affected by the timing of the peak detect signal applied toAND gate 306. The peak detector also turns itself off. Thus, externallogic timing is not critical. A further advantage of the presentinvention is that the current source drive provides superior noiserejection. Another advantage is that the accuracy of the peak detectoris determined by the comparator offset voltage. No matching timeconstants are required between channels. Yet another advantage of thepresent invention is that the MOSFET input comparator has minimal inputbias current. Thus, little charge is drained off from holding capacitorC102 by the inverting input of comparator 304. Still another advantageof the present invention is that there are no operational amplifierslew-rate/clamp problems. And yet another advantage is that the resetvoltage is always equal to or less than V_(REF) voltage.

FIG. 5 is a timing diagram illustrating peak detection according to thepresent invention. Between time zero and time T₁, the present inventionis operated in the hold mode where the output voltage has a nearlyconstant level of 2.7 volts indicated by a solid line. At time T₁, theinput signal 100 is coupled to the non-inverting input of comparator304. The input signal is indicated by a dotted line. As indicated inFIG. 5, the output voltage V_(OUT) (solid line) linearly increases untilthe instantaneous value of input signal 100 decreases below the outputvoltage V_(OUT). Thus, at time T₅, the input voltage causes the outputvoltage V_(OUT) to reach a maximum value of V_(MAX) (the peak voltage ofthe input signal).

CURRENT SINK WITH NEAR-GROUND COMPLIANCE RANGE

The present invention provides an improved voltage compliance currentsource for a differential transistor. This circuit is used in the peakdetector. As is well-known in the art, integrated circuits (IC)typically require a current source to provide an operating current for adifferential pair of transistors.

FIG. 6 is a diagram illustrating a prior art current source for adifferential pair. The first terminals of resistors R102 and R104 arecoupled to supply voltage V_(CC). The second terminals of resistors R102and R104 are coupled to the collectors of NPN transistors Q102 and Q104,respectively. The bases of transistors Q102 and Q104 receive inputsignals. The emitters of transistors Q102 and Q104 are coupled togetherto a first terminal of current source I102. The second terminal ofcurrent source I102 is coupled to ground. Current source I102 sinksconstant current I_(S).

The current source I102 illustrated in FIG. 6 is implemented in theprior art as illustrated in FIG. 7. Current source I102 comprises NPNtransistor Q202, resistor R202 and voltage source 206. The collector oftransistor Q202 is coupled to the emitters of transistors Q102 and Q104.The base of transistor Q202 is coupled to voltage source 206 having avalue V_(BIAS). Transistor Q202 is emitter-coupled through resistor R202to ground.

A disadvantage of the prior art scheme illustrated in FIG. 7 is theconstraint on the lower range of voltage compliance for current sourceI102. The voltage drop V_(RES) across resistor R202 is approximatelyequal to one-half of the base emitter voltage of transistor Q202. For anominal base-emitter voltage V_(BE) of 0.6 volts, the voltage V_(RES)across resistance R202 is approximately 0.3 volts. Accordingly, thelower limit of voltage compliance at the collector of transistor Q202 isapproximately 0.4 volts (V_(RES) +0.1 volts). This results in arestriction on the common-mode voltage at the bases of transistors Q102and Q104. A lower limit for common-mode voltage is desired that is lessthan the sum of 0.4 V plus the V_(BE) of transistor Q102 or Q104.

FIG. 8 illustrates an improved current sink for use with a differentialpair according to the present invention. The differential pair is usedin the comparator circuit of the present invention. The current sink ofthe present invention eliminates the emitter resistance voltage drop ofthe prior art shown in FIG. 7. It achieves a near-ground compliancerange.

In FIG. 8, a first terminal of diode 302 is coupled to supply voltageV_(CC). A second terminal diode 302 is coupled to first terminals ofresistors R102 and R104, and to the base of PNP transistor Q302. Secondterminals of resistors R102 and R104 are coupled to the collector oftransistors Q102 and Q104, respectively. Input signals are provided tothe bases of transistors Q102 and Q104. The emitters of transistors Q102and Q104 are coupled together to the collector of transistor Q202. Theemitter of transistor Q202 is coupled to ground. The emitter oftransistor Q302 is coupled to V_(CC). The collector of transistor Q302is coupled to the non-inverting input of a Norton amplifier U306. Areference current I302 having a constant value I_(REF) is provided tothe inverting input of Norton amplifier U306. The output of Nortonamplifier U306 is provided to the base of transistor Q202.

The value of collector current of transistor Q202 is sampled at the topof the differential pair (comprising transistors Q102 and Q104) andmirrored at the collector of transistor Q302. The collector oftransistor Q302 is coupled to the non-inverting input of Nortonamplifier U306. The inverting input of Norton amplifier U306 is coupledto reference current I302. As is well-known in the art, a Nortonamplifier is a current differencing amplifier that takes two currents asinputs, and provides a voltage output corresponding to the differencebetween the input currents.

When the collector current of transistor Q302 is greater than thereference current I302, Norton amplifier U306 produces a negative errorvoltage. The error voltage decreases the base voltage of transistor Q202until the current through transistor Q302 matches current referenceI302. When the current through transistor Q302 is less than thereference current I302, the output voltage of Norton amplifier U306increases. This increases the current sunk by transistor Q202 until itmatches I302. Near-ground compliance is achieved by eliminating resistorR202 found in FIG. 7 and high output impedance is realized by using afeedback loop. Thus, the present invention replaces series feedbackresistance R202 of the prior art with the feedback loop of Nortonamplifier U306, achieving near-ground compliance.

PREFERRED EMBODIMENT OF THE PRESENT INVENTION

FIGS. 9(a) and 9(b) are detailed diagrams illustrating the preferredembodiment of the present invention. The circuit is a closed-loop peakdetector comprising a current sink having near-ground voltagecompliance. The device comprises switching block 302, control block 320,difference amplifier 980, level shift stage 984, second amplificationstage 986, bias circuit 982, current sources 972 and 974, and capacitorC102. The current sink having near ground voltage compliance comprisesNorton amplifier 306 and current mirror 970. Difference amplifier 980and level shift stage 984 correspond to comparator 304 of FIG. 3A.Similarly, current source 974 and current sink 972 in conjunction withsecond amplification stage 986 correspond to current sources I310 andI312 of FIG. 3A, respectively.

Control block 320 comprises inverters I902 and I904, NAND gate A902, andFETs M906, M908 and M910. The RESET signal is coupled to the input ofinverter I902. The output of inverter I902 is coupled to a first inputof NAND gate A902 and to the gates of FETs M908 and M910. Current sink972 contains FETs M908 and M910. The PKDET signal is provided to thesecond input of NAND gate A902 of control block 320. The output of NANDgate A902 is coupled to the input of inverter I904 and to switchingblock 302. Specifically, the output of NAND gate A902 is coupled to thenon-inverting control inputs of transmission gates T904 and T908, aswell as the inverting control inputs of transmission gates T902 andT906. The output of inverter I904 is coupled to the gate of PMOS FETM906 and to switching block 302. Specifically, the output of inverterI904 is coupled to the non-inverting control inputs of transmissiongates T902 and T906, as well as the inverting control inputs oftransmission gates T904 and T908. PMOS FET M906 of control block 320 iscontained within current source 974.

Switching block 302 comprises analog transmission gates T902-T908. Theinput of transmission gate T904 is coupled to ground. Input signal 100is provided to the input of transmission gate T902. The output oftransmission gate T902 is coupled to the output of transmission gateT904 and to the input of transmission gate T906. Reference voltageV_(REF) is provided to the input of transmission gate T908. The outputsof transmission gates T906 and T908 are coupled to the gate of FET M902of difference amplifier 980.

Difference amplifier 980 includes FETs M902 and M904 that are the inputsto difference amplifier 980. The drains of FETs M902 and M904 arecoupled to supply voltage VCC. The source of FET M902 is coupled to thebase of transistor Q908 and to the collector of transistor Q912.Similarly, the source of FET M904 is coupled to the base of transistorQ910 and to the collector of transistor Q914. Transistors Q908 and Q910are a differential pair having a common emitter. Resistors 920 and 922are coupled between ground and the emitters of transistors Q912 andQ914, respectively. The bases of transistors Q912 and Q914 are coupledto the bases of transistors Q916-Q922.

The collectors of transistors Q908 and Q910 are coupled to the emittersof transistors Q904 and Q906, respectively. The bases ofcascode-connected transistors Q904 and Q906 are coupled to the emitterof transistor Q932 of biasing circuit 982. The collectors of transistorsQ904 and Q906 are coupled to first terminals of resistors R902 and R904,respectively. The collectors of transistors Q904 and Q906 provide thevoltage outputs of difference amplifier 980, and are coupled to thebases of transistors Q938 and Q940 of level shift stage 984,respectively. The second terminals of resistors R902 and R904 arecoupled together, and are coupled to supply voltage VCC throughdiode-connected PNP transistor Q902.

Diode-connected transistor Q902 corresponds to diode D302 of FIG. 8. Thebase of transistor Q902 is coupled to the collector of transistor Q902and to the base of PNP transistor Q934. The collector of transistor Q902is coupled to the second terminals of resistors R902 and R904 ofdifference amplifier 980. The emitter of transistor Q902 is coupled tosupply voltage V_(CC). The current I_(AMP) through difference amplifier980 is mirrored by PNP transistor Q934 at the collector ofdiode-connected transistor Q902. PNP transistor Q934 sources a currentI_(IN) from its collector to Norton amplifier 306 (described below). Theemitter of transistor Q934 is coupled to supply voltage V_(CC). PNPtransistor Q934 corresponds to transistor Q302 of FIG. 8.

In FIG. 9(a), biasing circuit 982 is indicated within a dashed line. Thecollector of transistor Q932 is coupled to supply voltage V_(CC). Thebase of transistor Q932 is coupled to the base of transistor Q936.Transistor Q936 is diode-connected. The base of transistor Q936 iscoupled to the collector of transistor Q936. Resistor R914 is coupledbetween supply voltage V_(CC) and the collector of transistor Q936. Theemitter of transistor Q936 is coupled to a first terminal of resistorR910. The second terminal of resistor R910 is coupled to the base of NPNtransistor Q924 and the collector of NPN transistor Q918. The collectorof transistor Q924 is coupled to supply voltage V_(CC). The emitter oftransistor Q924 is coupled to the bases of transistors Q912 throughQ922. Transistors Q916 and Q918 form a current mirror. Resistors R924and R926 are coupled between ground and the emitters of transistors Q916and Q918, respectively. The collector of transistor Q916 is coupled tothe collector of transistor Q928.

PNP transistor Q928 is diode-connected. The base of transistor Q928 iscoupled to its collector. The base of PNP transistor Q928 is coupled tothe bases of PNP transistors Q926 and Q930. PNP transistor Q926 sourcesa bias current I_(BIAS) from its collector to current mirror 970(described below). Resistor R906 is coupled between the emitter of NPNtransistor Q932 and the emitter of PNP transistor Q926. Similarly,resistors R908 and R912 are coupled between the emitter of transistorQ932 and the emitters of PNP transistors Q928 and Q930, respectively.The collector of PNP transistor Q930 sources reference current I_(REF)to Norton amplifier 306 (described below). Biasing circuit 982 providesa bias voltage to cascode-connected transistors Q904 and Q906 ofdifference amplifier 980. Also, it provides another bias voltage totransistors Q920 and Q922 of level shift stage 984. Further, biasingcircuit 982 provides reference and bias currents I_(REF) and I_(BIAS) toNorton amplifier 306 and current mirror 970.

Level shift stage 984 receives output signals from difference amplifier980. Specifically, the bases of transistors Q938 and Q940 are coupled tothe collectors of cascode-connected transistors Q904 and Q906,respectively. The collectors of NPN transistors Q938 and Q940 arecoupled to supply voltage V_(CC). The emitters of transistors Q938 andQ940 are coupled to diode-connected transistors Q942 and Q944,respectively. The bases of transistors Q942 and Q944 are each connectedto the collector of transistors Q942 and Q944, respectively. Theemitters of transistors 942 and 944 are coupled to the collectors oftransistors Q920 and Q922, respectively. Resistors R928 and R930 arecoupled between ground and the emitters of transistors Q920 and Q922,respectively. The emitters of diode-connected transistors Q942 and Q944of level shift stage 984 are coupled to the bases of transistors Q952and Q950 of second amplification stage 986, respectively. Thus, theemitters of transistors Q942 and Q944 of level shift stage 984 provideoutput signals to second amplification stage 986.

Second amplification stage 986 comprises a differential pair Q950 andQ952. The emitters of transistors Q950 and Q952 are coupled together tothe collector of transistor Q954. A resistor R932 is coupled between theemitter of transistor Q954 and ground. Transistor Q954 is biased byvoltage V_(PKBIAS) coupled to the base of transistor Q954. TransistorQ954 and resistor R932 are a conventional prior art current source. Thecollector current of transistor Q952 of second amplification stage 986controls the current I_(SINK) of current sink 972. Similarly, thecollector current of transistor 950 of second amplification stage 986controls current I_(SOURCE) of current source 974.

Current sink 972 is coupled to the collector of transistor 952 of secondamplification stage 986. Specifically, the collector of PNP transistorQ946 is coupled to the collector of NPN transistor Q952 of secondamplification stage 986. Resistor R938 is coupled between supply voltageV_(CC) and the emitter of transistor Q946. A first terminal of resistorR942 is coupled to the base of transistor Q946. The second terminal oftransistor R942 is coupled to the collector of transistor Q946 and tothe base of transistor Q948. Resistor R940 is coupled between supplyvoltage V_(CC) and the emitter of transistor Q948. Thus, a current issourced from the collector of transistor Q948 that is proportional tothe current sunk by transistor Q952 of second amplification stage 986.

The collector of transistor Q948 is coupled to the collector oftransistor Q956 and to the base of transistor Q958. The collector oftransistor Q958 is coupled to supply voltage V_(CC). Resistor R934 iscoupled between the emitter of transistor Q956 and ground. The emitterof transistor Q958 is coupled to the bases of transistors Q956 and Q960.Resistor R936 is coupled between ground and the emitter of transistorQ960. Transistors Q956 and Q960 form a current mirror. Thus, transistorQ960 sinks current I_(SINK) that is proportional to the current sourcedfrom the collector of transistor Q946. The collector of transistor Q960of current sink 972 is coupled to the FET input M904 (gate) ofdifference amplifier 980, current source 974, and capacitor C102.

Current sink 972 further includes FETs M908 and M910 of control block320. The drain of FET M908 is coupled to the base of transistor Q958.The gates of FETs M908 and M910 are coupled together to the output ofinverter I902. The drain of FET M910 is coupled to the emitter oftransistor Q958 and to the bases of transistors Q956 and Q960. Thesources of FETs M908 and M910 are coupled to ground.

The collector of PNP transistor Q962 of current source 974 is coupled tothe collector of NPN transistor Q950 of second amplification stage 986.Resistor R940 is coupled between supply voltage V_(CC) and the emitterof PNP transistor Q962. A first terminal of resistor R948 is coupled tothe base of transistor Q962. The second terminal of resistor R948 iscoupled to the collector of transistor Q962 and the base of PNPtransistor Q964. Resistor R946 is coupled between supply voltage VcC andthe emitter of transistor Q964. Transistors Q962 and Q964 of currentsource 974 form a current mirror.

Current source 974 includes FET M906 of control block 320. The output ofinverter I904 of control block 320 is coupled to the gate of FET M906.The drain of FET M906 is coupled to the base of transistor Q964. Thesource of FET M906 is coupled to supply voltage VcC. Capacitor C102 iscoupled between the collector of transistor 964 and ground. TransistorQ964 of current source 974 sources an output current I_(SOURCE) that isproportional to the current sunk by transistor Q950 of secondamplification stage 986.

The collector of PNP transistor Q930 of biasing circuit 982 sources acurrent I_(REF) to Norton amplifier 306. Specifically, the collector oftransistor Q930 is coupled to the collector of NPN transistor Q976.Transistor Q976 is diode-connected. The base of transistor Q976 iscoupled to the collector of transistor Q976 and to the base oftransistor Q974. The emitters of transistors Q974 and Q976 are coupledto ground. A first terminal of capacitor C902 is coupled to the bases oftransistors Q974 and Q976. A second terminal of capacitor C902 iscoupled to the collector of transistor Q974 and to the base oftransistor Q972. Resistor R918 is coupled between the emitter oftransistor Q972 and ground. The collector of transistor Q934 sources acurrent I_(IN) to the collector of transistor Q974, the base oftransistor Q972 and the second terminal of capacitor C902. CurrentI_(IN) is proportional to the current through difference amplifier 980.Norton amplifier receives input current I_(IN) from transistor Q934 andreference current I_(REF) from transistor Q930 of biasing circuit 982.

The collector of transistor Q972 provides the output of Norton amplifier306 which is coupled to the collector of transistor Q970 of currentmirror 976. Transistor Q926 of biasing circuit 982 sources bias currentI_(BIAS) to current mirror 976 and the output of Norton amplifier 306.The collector of transistor Q926 of biasing circuit 982 is coupled tothe collector of transistor 72, the base of transistor Q966, and thecollector of transistor Q970. The collector of transistor Q966 ofcurrent mirror 976 is coupled to supply voltage V_(CC). The emitter oftransistor Q966 is coupled to the bases of transistors Q968 and Q970, aswell as a first terminal of resistor R916. The emitters of transistorsQ968 and Q970, as well as the second terminal of resistor R916, arecoupled to ground. The collector of transistor Q968 of current mirror976 is coupled to the emitters of transistors Q908 and Q910 ofdifference amplifier 980. Transistor 968 sinks constant current I_(AMP).Current mirror 976 replaces transistor Q202 of FIG. 8.

As illustrated in FIG. 9(a), the PKDET and RESET signals are provided tothe control block 320 of the present invention. The closed-loop peakdetector operates in three modes: reset, peak detect and hold. The peakdetector comprises a difference amplifier 980 having a non-invertinginput (FET M902) coupled through switching block 302 to input signal 100(which can be a full wave rectified signal) or a reference voltageV_(REF). The output voltage V_(OUT) across capacitor C102 is coupled ina feedback loop to the inverting input (FET M904) of differenceamplifier 980. Transistors Q904 and Q906 of difference amplifier 980 arebiased by the voltage at the emitter of transistor Q932 of biasingcircuit 982. The cascode configuration of transistors Q904 and Q906 withdifferential pair Q908 and Q910 provides high output impedance at thecollectors of transistors Q904 and Q906. Also, it prevents undesiredhigh frequency feedback in transistors Q908 and Q910.

Current mirror 976 sinks a constant current I_(AMP) through transistorsQ908 and Q910. Imbalances between the voltages applied to the gates ofFETs M902 and M904 steer currents through transistors Q908 and Q910.This produces a voltage difference between the collector voltages ofcascode transistors Q904 and Q906 that is output to the level shiftstage 984. When the gate voltages of FETs M902 and M904 are equal,current I_(AMP) divides evenly (I_(AMP) /2) through transistors Q908 andQ910. This produces equal voltages applied to the bases of transistorsQ938 and Q940 of level shift stage 984.

Biasing circuit 982 provides bias voltages to difference amplifier 980and level shift stage 984. It also provides currents I_(REF) andI_(BIAS) to Norton amplifier 306 and current mirror 976. Diode-connectedtransistors Q936 and Q918 and resistors R910, R914 and R926 establish astable operating current in biasing circuit 982. The base voltage oftransistor Q918 biases proportional currents through transistors Q912and Q914 of difference amplifier 980, Q920 and Q922 of level shift stage984, and Q916 of biasing circuit 982. Transistor Q932 produces anemitter voltage for biasing cascode-connected transistors Q904 and Q906of difference amplifier 980. The emitter voltage of transistor Q932 alsoestablishes a known voltage for other current sources in biasing circuit982.

Biasing circuit 982 comprises another current mirror circuit containingtransistor Q928. Transistor Q916 produces a known current throughtransistor Q928 and resistor R908, thereby producing known currentsI_(REF) and I_(BIAS) through transistors Q930 and Q926. The currentsI_(REF) and I_(BIAS) are generated in response to the base voltage oftransistor Q928 applied to the bases of transistors Q926 and Q930. Thecollector of transistor Q930 sources current I_(REF) to Norton amplifier306. Similarly, the collector of transistor Q926 sources currentI_(BIAS) to current mirror 976.

Transistor Q934, Norton amplifier 306 and constant current I_(BIAS) forma feedback loop used to maintain a constant current I_(AMP) sunk bycurrent mirror 976 having near-ground voltage compliance.Diode-connected transistor Q902 samples operating current I_(AMP) in thedifference amplifier 980. In turn, transistor Q934 sources a currentI_(IN) to Norton amplifier 306 that is proportional to I_(AMP), Nortonamplifier 306 receives reference current I_(REF) from biasing circuit982. Norton amplifier 306 mirrors current I_(REF) (through transistorQ976) in transistor Q974. Capacitor C902 provides frequency stabilitysince Norton amplifier 306 is contained in the feedback loop of thecurrent sink having near-ground compliance in the present invention. Thedifference in current between current I_(IN) (sourced by transistorQ934) and the fixed current I_(REF) through transistor Q974 biasestransistor Q972. Transistor Q972 is emitter degenerately configured(R918).

The difference between currents I_(IN) and I_(REF) modulates the currentI_(AMP) sunk by current mirror 976. Changes in the bias current totransistor Q972 varies the collector current of transistor Q972 anddraws current ΔI from the fixed current I_(BIAS) sourced by PNPtransistor Q934 (typically, current I_(BIAS) is 20% larger than currentI_(REF) when Norton amplifier is not connected to current mirror 976).Transistor Q966 biases the bases of transistors Q968 and Q970. Thecurrent (I_(BIAS) -ΔI) through transistor Q970 determines currentI_(AMP) sunk by transistor 968. Thus, the output of Norton amplifier 306modulates the current I_(AMP) sunk by current mirror 976 in response todifferences in current between currents I_(IN) and I_(REF1). CurrentI_(IN) is sourced in response to current I_(AMP) through differenceamplifier 980 sampled by transistor Q902.

The base voltage of transistor Q918 biases transistors Q920 and Q922 oflevel shift stage 984 to produce fixed, equal currents throughtransistors Q920 and Q922. The collector voltages of transistors Q904and Q906 are applied to the bases of transistors Q938 and Q940. Levelshift stage 984 translates down the collector voltages of transistorsQ904 and Q906 by two base-emitter drops (2 V_(BE)) at the emitters ofdiode-connected transistors Q942 and Q944. The emitter voltages oftransistors 942 and 944 of level shift stage 984 bias the bases ofdifferential pair transistors Q952 and Q950 of second amplificationstage 986, respectively. Transistor Q954 is biased by bias voltageV_(PKBIAS) to produce a constant collector current sunk by transistorQ954. Bias voltage V_(PKBIAS) may be a voltage received from adigital-to-analog converter (DAC) to set the collector current oftransistor Q954.

The constant collector current of transistor Q954 is sunk from theemitters of transistors Q952 and Q950 according to the difference inemitter voltages of transistors Q942 and Q944, respectively. A currentproportional to the collector current of Q952 is sourced by thecollector of Q948 of the current mirror comprising PNP transistors Q946and Q948. The collector current of transistor Q952 is provided to acurrent mirror comprising transistors Q956 and Q960. The emitter oftransistor Q958 biases transistors Q956 and Q960. The collector oftransistor Q960 of current sink 972 is coupled to capacitor C102. Thecollector current of transistor Q948 is mirrored by transistors Q956 andQ960 to produce current I_(SINK).

The drain of FET M908 is coupled to the base of transistor Q958.Similarly, the drain of FET M910 is coupled to the bases of transistorsQ956 and Q960. When RESET signal is low, the output of inverter I902 ofcontrol block 320 is high. The high output of inverter I902 enables FETsM908 and M910, thereby pulling the bases of transistors Q956, Q958 andQ960 to ground. This disables the output current mirror comprisingtransistors Q956 and Q960. The output (collector of transistor Q960) ofcurrent sink 972 is put into a low leakage current state. When RESETsignal is high, current sink 972 operates as described above. TransistorQ960 sinks I_(SINK) in response to base voltage differences betweentransistors Q952 and Q950 of second amplification stage 986.

A current proportional to the collector current of Q950 is sourced bythe collector of transistor Q964 of the current mirror comprising PNPtransistors Q962 and Q964. The collector of transistor Q964 of currentsource 974 is coupled to capacitor C102. The collector of transistorQ964 sources current I_(SOURCE) to capacitor C102. Transistor Q964sources I_(SOURCE) in response to base voltage differences betweentransistors Q952 and Q950 of second amplification stage 986.

The drain of PMOS FET M906 is coupled to the base of transistor Q964.When RESET signal is high and/or the PKDET signal is low, the output ofinverter I904 of control block 320 is low. The low output of inverterI904 enables FET M906, thereby pulling the base of transistor Q964 up tosupply voltage V_(CC). This disables transistor Q964. The output(collector of transistor Q964) of current source 974 is put into a lowleakage current state. When RESET signal is low and the PKDET signal ishigh, current sink 974 operates as described above.

As illustrated in FIGS. 9(a) and 9(b), the PKDET and RESET signals areprovided to the peak detector circuit of the present invention. Thecircuit operates in three modes: reset, peak detect and hold. The PKDETand RESET signals control operation of switching block 302 and currentsink 972 and source 974. Table 1 lists the input voltage V_(IN) (gate ofFET M902) of differential amplifier 980 and the states of current sink972 and source 974 in response to the PKDET and RESET signals.

                  TABLE 1                                                         ______________________________________                                        Mode  PKDET    RESET    V.sub.IN                                                                              Sink 972                                                                             Source 97A                             ______________________________________                                        hold  0        0        V.sub.REF                                                                             Disabled                                                                             Disabled                               peak  1        0        Input   Disabled                                                                             Enabled                                detect                  Signal 100                                            reset 0        1        V.sub.REF                                                                             Enabled                                                                              Disabled                               don't 1        1        V.sub.REF                                                                             Enabled                                                                              Disabled                               care                                                                          ______________________________________                                    

As indicated in Table 1, the RESET signal overrides the PKDET signal.Thus, when both signals are logic high, the closed-loop peak detector ofFIGS. 9(a) and 9(b) is kept in the reset mode.

Referring to FIG. 9(a), when in the reset mode, switching block 302couples the gate of FET M902 of difference amplifier 980 to referencevoltage V_(REF). Current source 974 is disabled and current sink 972 isenabled. When output voltage V_(OUT) is greater than voltage referenceV_(REF), a positive voltage difference is produced between thecollectors of transistors Q904 and Q906. This difference in collectorvoltages sinks a larger current through transistor Q952 than throughtransistor Q950. However, in reset mode, current source 974 is disabledin a low leakage current state. Accordingly, a proportional currentI_(SINK) is sunk by current sink 972 to discharge capacitor C102. Thelow output of inverter I902 enables current sink 972. Current sink 972continues to draw charge from capacitor C102 until the output voltageV_(OUT) of the peak detector is discharged to a value slightly less thanthe baseline voltage V_(REF) applied to the gate of FET M902. When thisoccurs, the difference in collector voltages of transistors Q904 andQ906 output by difference amplifier 304 becomes negative. This maintainsthe output voltage V_(OUT) at the constant baseline level of V_(REF).

When in the peak detection mode, the PKDET signal is high and the RESETsignal is low. Input signal 100 is provided to FET M902 by switchingblock 302. The output of inverter I902 of control block 320 is high,thereby disabling current sink 972. When input signal 100 is lower thanthe voltage V_(OUT) stored across capacitor C102, the positive voltagedifference across the collectors of transistors Q904 and Q906 reducesthe current through transistor Q950 of second amplification stage 986 tozero. In turn, the current mirrored by transistor Q964 becomes zero,thereby preventing charging of capacitor C102. When input signal 100exceeds output voltage V_(OUT), the negative voltage difference acrossthe collectors of transistors Q904 and Q906 switches on the currentthrough transistor Q950 of second amplification stage 986. Currentsource 974 charges capacitor C102 until the input signal 100 decreasesbelow the instantaneous level of output voltage V_(OUT). Differenceamplifier 980 has a MOSFET input comparator that does not require biascurrent. It compares the input signal 100 with the voltage V_(OUT) usingclosed loop feedback.

When the voltage difference across the collectors of transistors Q904and Q906 is negative, current source 974 provides current I_(SOURCE) tocharge capacitor C102 until the output voltage V_(OUT) is equal to theinstantaneous value of the input signal 100. The output voltage V_(OUT)across capacitor C102 is slew-rate limited. Therefore, the presentinvention requires a series of positive lobes of input signal 100 topump up voltage V_(OUT) to the most positive value of input signal 100.Thus, in peak detect mode, current source 974 successively chargescapacitor C102 when input signal 100 exceeds voltage V_(OUT), therebyincreasing voltage V_(OUT).

In the hold mode of the present invention, the PKDET and RESET signalsare both low. In this mode, switching block 302 couples referencevoltage V_(REF) to FET M902 of difference amplifier 980. With the PKDETand RESET signals low, current sink 972 and source 974 are disabled.Thus, current is not sourced to or drained from capacitor C102, andcapacitor C102 maintains a nearly constant amount of charge. Thus, theoutput voltage V_(OUT) is equal to the peak voltage of the input signal100 stored in capacitor C102.

TIMING DIAGRAM ILLUSTRATING THREE MODES

FIG. 4 is a timing diagram illustrating the input voltage applied to FETM902 of difference amplifier 980 and the corresponding output voltagesignal V_(OUT). The input voltage at FET M902 is indicated by a dottedline. During times 0 to T₁, the PKDET signal is low and the RESET signalis high. Initially, capacitor C102 is discharged and, thus, the outputsignal V_(OUT) has a level of 0 volts (indicated by a solid line).Because switching block 302 provides voltage V_(REF) to the FET M902,the input voltage V_(IN) is equal to V_(REF). As indicated in FIG. 4,current source 974 is disabled. Current sink 972 does not sink currentbecause current I_(AMP) is steered through transistor 950 of secondamplification stage 986. The output voltage V_(OUT) is maintained nearground.

Between times T₁ and T₃, the present invention operates in the peakdetect mode. As illustrated in FIG. 3B, the PKDET signal is high and theRESET signal is low. In this mode, FET M902 is coupled to input signal100. In FIG. 4, the input signal 100 (indicated by a dashed line) is afull-wave rectified voltage signal. The diagram illustrates six lobes ofinput signal 100 during this interval.

At time T₁, the output voltage V_(OUT) is equal to 0 volts. Betweentimes T₁ and T₂, the input signal 100 provided to FET M902 is greaterthan the output signal V_(OUT) provided to FET M904 This produces alarger voltage at the collector of transistor Q906 than Q904 ofdifference amplifier 980. Current source 974 charges capacitor C102 inresponse to the current sunk by transistor Q950. In FIG. 4, the outputvoltage V_(OUT) increases between times T₁ and T₂ in response to theinput signal 100.

At time T₂, the instantaneous voltage V_(OUT) rises to the lowest levelof input signal 100. Difference amplifier 980 produces a positivevoltage difference at the collectors of transistors Q904 and Q906. Thisturns off current source 974 (current sink 972 is disabled by the outputof inverter I902). Because current sink 972 and source 974 are disabled,the output voltage V_(OUT) of the peak detector is held at a constantlevel. The output voltage V_(OUT) remains constant until the voltage ofinput signal 100 applied to difference amplifier exceeds the outputvoltage V_(OUT). Between times T₂ and T₃, the output voltage V_(OUT)periodically increases as capacitor C102 is charged by current source974.

At time T₃, the peak detector is switched to the hold mode by settinglow the PKDET and RESET signals. Both current sink 972 and source 974(I_(SINK) and I_(SOURCE)) are disabled. During the hold mode, switchingblock 302 is coupled to constant voltage reference V_(REF) as indicatedby the constant level (indicated by a dotted line). In FIG. 4, betweentimes T₃ and T₄, the output voltage V_(OUT) is held at a constant levelV₁ (where voltage 0≦V₁ ≦V_(MAX)).

At time T₄, the peak detector of the present invention operates in thereset mode. The PKDET signal is low and the RESET signal is high.Setting the PKDET signal to zero disables current source 974. Switchingblock 302 couples FET M902 of differential amplifier 980 to thereference voltage V_(REF). As indicated in FIG. 4, the output voltageV_(OUT) (solid line) has a value of V₁. It is greater than the inputvoltage V_(IN) of differential amplifier 980 (indicated by a dottedline) equal to V_(REF). Differential amplifier 980 produces a negativecollector voltage difference across transistors Q904 and Q906, therebyproducing current I_(SINK) that discharges capacitor C102. The outputvoltage V_(OUT) decreases from a level of V₁ to V_(REF).

At time T₅, V_(OUT) reaches the reference voltage V_(REF) coupled todifferential amplifier 980. In turn, the output of differentialamplifier 980 disables current source 974. Between times T₅ and T₆, theoutput voltage V_(OUT) has a constant level of V_(REF).

At time T₆, the present invention operates in the peak detect mode.Between times T₆ and T₇, input signal 100 is coupled to FET M902 ofdifferential amplifier 980. In FIG. 4, there are six lobes of the inputsignal 100 during this interval. When input signal 100 exceeds theoutput signal V_(OUT), differential amplifier 980 produces a negativecollector voltage difference across transistors Q904 and Q906. Inresponse, current source 974 charges capacitor C102 until the inputsignal 100 decreases below the instantaneous value of the output voltageV_(OUT). Prior to time T₇, current source 974 charges hold capacitorC102 to the peak value of the input signal 100 (as illustrated in FIG.4).

At time T₇, the present invention is operated in the hold mode where thePKDET and RESET signals are low. This disables current source 974 andsink 972 which prevents the sourcing and sinking of current to holdingcapacitor C102. Thus, in FIG. 4, the output voltage V_(OUT) remains atits peak value of V_(MAX) after time T₇.

The present invention provides closed loop peak detection with lowoffset voltage, no amplifier slew-rate limitations, and with enhancednoise rejection capability. The peak voltage stored on holding capacitorC102 is not affected by the timing of the peak detect signal applied tocontrol block 320. The peak detector also turns itself off. Thus,external logic timing is not critical. A further advantage of thepresent invention is that the current source drive provides superiornoise rejection. Another advantage is that the accuracy of the peakdetector is determined by the comparator offset voltage. No matchingtime constants are required between channels. Yet another advantage ofthe present invention is that the MOSFET input comparator has a minimalinput bias current. Thus, little current is drained off from holdingcapacitor C102 by the FET M904 input of difference amplifier 980. Stillanother advantage of the present invention is that there are nooperational amplifier slew-rate/clamp problems. And yet anotheradvantage is that the reset voltage is always equal to or less thanV_(REF) voltage.

In this manner, a closed-loop peak detector is disclosed.

I claim:
 1. A circuit comprising:a diode coupled to a supply voltage;current sinking means coupled to ground; a differential pair, having acircuit input and output, coupled between said diode and said currentsinking means; a transistor means coupled to said differential pair andsaid diode for detecting a first current provided to said currentsinking means, said transistor means providing a second current inresponse to said first current; Norton amplification means coupled tosaid current sinking means for receiving said second current and areference current, said Norton amplification means controlling saidfirst current through said current sinking means in response to saidsecond current and said reference current.
 2. The circuit of claim 1wherein said current sinking means comprises an NPN transistor forsinking said first current.
 3. The circuit of claim 1 wherein saidcurrent sinking means comprises current mirroring means for sinking saidfirst current.